12b: In the above reaction a single starting material can form two stereoisomers

Question

12b: In the above reaction a single starting material can form two stereoisomers, but yields one of them preferentially. Any reaction in which this occurs is defined as _______________. The reduction of the ketone shown below with sodium borohydride results in the following product distribution. Explain why the first product is formed in such a larger amount than the second. Hint: remember that the carbon of the carbonyl group is sp2 hybridized and planar and that the reduction involves a hydride transfer from NaBH4 to the carbon of the carbonyl group. In the above reaction a single starting material can form two stereoisomers, but yields one of them preferentially. Any reaction in which this occurs is defined as .

Explanation / Answer

a) If attack at the top occurs then the geminal methyl groups(attached to the same carbon) would offer steric hindrance to its interaction.

So the least hindered endo attack by hydride at the C-atom predominate that of exo attack.

So the product formed is that of an endo attack.

NaBH4 attacks via cyclic transition state from the bottom .

https://drive.google.com/file/d/0B2cLQie7vE64RVBBdnJLTklLZGJTLWd0Y1JYTVBqSVNJSWsw/view?usp=sharing

see image.

B)Stereoselective reaction- when one product predominates in any reaction due to steric reasons

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